You’re not misunderstanding, we could add this option to the category/tag contexts as well. Maybe we will revisit this later, but for now, users have several ways to search by username in a category/tag context: username (without @) will show both options and globally they can do @username #category from anywhere in the site. The in-topic search is special, the only way to show posts as results there is to use the suggestions.
Marking this for closing since the topic search scenario has been fixed in this PR.